THE MEASUREMENT AND ANALYSIS OF BATHYMETRY, MORPHOLOGY, AND SEDIMENT THICKNESS

Transcription

1 THE MEASUREMENT AND ANALYSIS OF BATHYMETRY, MORPHOLOGY, AND SEDIMENT THICKNESS Ron Macnab Geological Survey of Canada (Retired)

2 OUTLINE OF PRESENTATION Principles and practice of: Echo-sounding Seismic reflection and refraction Database and GIS tools Test of Appurtenance Foot of Slope 2500 metre isobath Gardiner Line Case studies and examples Discussion, Q & A

3 MEASURING THE DEPTH OF WATER: PRINCIPLES, PRACTICES, AND PROBLEMS In the Article 76 context, water depth is necessary: To locate the Foot of the Slope (primarily a morphological exercise) To locate the 2500 m Isobath (primarily a bathymetric exercise)

4 THE BLUE BOOK Continental Shelf Limits: the Scientific and Technical Interface Chapter 9 (Tony Laughton & Steve Shipman) Historical Methods of Depth Measurement Chapter 10 (John Hughes Clarke) Present-Day Methods of Depth Measurement Chapter 11 (David Monahan) Interpretation of Bathymetry

5 IHO MANUAL ON HYDROGRAPHY

6 THE MEASUREMENT OF DEPTH IS ACTUALLY A MEASUREMENT OF ELAPSED TIME

7 THE VELOCITY OF SOUND IN SEAWATER IS VARIABLE Function of Temperature, Salinity, Pressure Varies from the sea surface to the sea floor Varies from one location to another Varies from one season to another These variations must be taken into account because they can introduce significant errors in the measurement of depth

8 CORRECTING FOR VARIATIONS IN SOUND VELOCITY Refer to published Tables of Corrections Measure sound velocity in situ: Observations of Temperature, Salinity, Pressure throughout the water column Direct measurement of velocity profile Acoustical techniques

9 POSITIONING TECHNIQUES IN MARINE SURVEYING Databases used for determining OCS limits may represent a mix of historic and modern observations. Most likely, these observations will have been collected with navigational systems that have evolved over the years. It is important to understand the principles and limitations of early navigation systems, in order to assess the suitability of historic observations for Article 76 purposes.

10 POSITIONING MARINE SURVEYS: HISTORIC METHODS I Discussed in Cook & Carleton, Chapter 7: Astronomic observations Intermittent Sun - accurate to 3 km Stars - accurate to 2 km Deduced reckoning (DR) Continuous - based on course & speed made good Subject to many and varied errors Land-based radionavigation Continuous accuracy within tens to hundreds of metres Circular measuring distances to known points Hyperbolic - measuring time differences between signals High frequency - short range, high accuracy Low frequency - long range, low accuracy

11 POSITIONING MARINE SURVEYS: HISTORIC METHODS II Discussed in Cook & Carleton, Chapter 7: Transit satellite Periodic Accurate to 100 m Inertial Integration of horizontal and vertical accelerations Subject to many and varied errors, particularly drift Integrated systems Combining best of DR, radionavigation, Transit, inertial Continuous Accuracy - tens to hundreds of m

12 POSITIONING MARINE SURVEYS: MODERN METHODS Discussed in Cook & Carleton, Chapter 8 - (GPS): Standard Positioning Service (SPS) civilian Coarse/Acquisition (C/A) code m hor, 150 m vert Selective Availability (SA) - C/A signal degraded SA removed May accuracy improved to m Precise Positioning Service (PPS) - encrypted, for military use Precise (P) code - 16 m Differential GPS Fixed reference station derives corrections for mobile receivers Accuracy 1-5 m Carrier phase measurements Future potential for real-time navigation to the cm level

13 MAPPING THE DEPTH OF THE SEA: ACOUSTIC METHODS Single beam Depths at single points beneath the vessel Sidescan sonar Bottom characteristics (not depths) to either side of vessel Multibeam Multiple depths at points beneath and to either side of the vessel

14 SINGLE (NARROW) BEAM MAPPING John Hughes Clarke, UNB

15 SINGLE (NARROW) BEAM WITH SIDESCAN SONAR John Hughes Clarke, UNB

16 MULTIBEAM MAPPING John Hughes Clarke, UNB

17 SINGLE (WIDE) BEAM VS MULTIBEAM

18 SEABED PORTRAYED FROM WIDE BEAM SOUNDINGS ALONG RANDOM TRACKLINES

19 SEABED PORTRAYED FROM WIDE BEAM SOUNDINGS ALONG RANDOM TRACKLINES SAME SEABED PORTRAYAL, ENHANCED WITH MULTIBEAM SOUNDINGS

20 HOW BEAMWIDTH AFFECTS OUR PERCEPTION OF THE SEABED IHO Manual on Hydrography

21 Hyperbolic Echoes Bottom Minimum Wavelength NOTE : as long as target below transducer, does not bias the shallowest depth estimation BUT : does significantly limit minimum resolvable horizontal Wavelength. A result of the projected beam width. John Hughes Clarke, UNB

22 Bottom Slope Effect Results in underestimation of bottom slopes Representation of a Trough (e.g.: dredged channel) Can have consequences for Dredge payment as underestimates Volume of sediment removed. John Hughes Clarke, UNB

23 Side Swipes Three profiles: 1. Directly over the target 2. Offset but over the target 3. Offset, never over the target Note true elevation generally underestimated And location imperfectly defined John Hughes Clarke, UNB

24 COMPARING NARROW- AND WIDE- BEAM PORTRAYALS OF THE SEABED IHO Manual on Hydrography

25 NARROW BEAMS = HIGHER RESOLUTION Simrad EM1000 (2.4 x 3.3 deg. beams) Simrad EM3000 (1.5 x 1.5* deg. beams) Depths: 25-45m Bedford Basin John Hughes Clarke, UNB

26 BATHYMETRY DERIVED FROM SATELLITE ALTIMETRY

27 WORLD BATHYMETRY FROM SATELLITE ALTIMETRY

28 BATHYMETRY DERIVED FROM SATELLITE ALTIMETRY Bathymetric maps derived from satellite altimetry cover most of the globe. They are very useful for presenting regional views and for general analysis. These maps cannot resolve seabed features to better than about 8 km, with depth inaccuracies of 100s of metres. The CLCS has declared that it will not accept such maps in support of submissions.

29 AEGIR RIDGE DEPTHS DERIVED FROM SATELLITE ALTIMETRY Peter Vogt, USNRL

30 AEGIR RIDGE DEPTHS MEASURED WITH MULTIBEAM DEPTHS DERIVED FROM SATELLITE ALTIMETRY Peter Vogt, USNRL

31 BATHYMETRY DERIVED FROM SEISMIC MEASUREMENTS Deemed acceptable by CLCS, but only as secondary data Conventional annotation - TWT Converted annotation - Depth John Hughes Clarke, UNB

32 MEASURING THE THICKNESS OF SEDIMENT: PRINCIPLES, PRACTICES, AND PROBLEMS In the Article 76 context, sediment thickness is needed to locate the Gardiner Line, i.e. the line where the thickness of sedimentary material is equal to 1% of the distance back to the foot of slope. Several considerations that apply to the measurement of bathymetry also apply to the measurement to sediment thickness, e.g. choice of frequency, acoustic propagation errors, etc.

33 THE BLUE BOOK Continental Shelf Limits: the Scientific and Technical Interface Chapter 12 (Kasuga, Nishizawa, Ohara, Kusunoki, and Katsura) Seismic Reflection and refraction Methods

34 A PASSIVE CONTINENTAL MARGIN Wood et al. 2003

35 Wood et al. 2003

36 THE SEISMIC REFLECTION TECHNIQUE MEASURES THE TWO-WAY TRAVEL TIMES OF SOUND PULSES THROUGH SEDIMENT LAYERS THAT LIE BENEATH THE OCEAN FLOOR. BUT THOSE TRAVEL TIMES NEED TO BE COMBINED WITH THE SOUND VELOCITY FOR EACH LAYER IN ORDER TO YIELD VALUES OF SEDIMENT THICKNESS.

37 THE SEISMIC REFRACTION TECHNIQUE MEASURES THE HORIZONTAL TRAVEL TIMES OF SOUND PULSES THROUGH SEDIMENT LAYERS THAT LIE BENEATH THE OCEAN FLOOR. COMBINING THOSE TRAVEL TIMES WITH THE KNOWN DISTANCE BETWEEN SOUND SOURCE AND RECEIVER YIELDS THE SOUND VELOCITY FOR EACH LAYER.

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39 VELOCITY ANALYSIS CALCULATING SOUND VELOCITIES IN SEDIMENT Requires complex sensor configuration and extensive computer processing

40 MEASURING SEDIMENT THICKNESS FOR ARTICLE 76 Single channel system will suffice if only depth to acoustic basement is being sought this is cheap and simple to operate. Simple multichannel system will yield stratigraphic information in addition to depth to basement, but refraction experiments may be needed to calculate sound velocities this is more expensive and complex. Industry multichannel system will yield detailed cross-section of sediment down to the basement this is very expensive and may yield more information than is required.

41 GATHER NEW DATA, OR USE EXISTING DATA? Questions to ask when making the decision: Availability, coverage, and quality of existing holdings? Prospects for building a better article 76 case with new data? Time and effort to collect new data? Cost of collecting new data?

42 SOURCES OF EXISTING DATA

43 GEBCO (BLUE) AND IBC (RED) SHEETS

44 GLOBAL TRACKLINE HOLDINGS AT NGDC

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47 GLOBAL SEDIMENT THICKNESS GRID NGDC

48 BASIC SURVEY DESIGN GET EXPERT HELP! SINGLE BEAM MULTIBEAM IHO Manual on Hydrography

49 DATABASE AND GIS TOOLS Proprietary tools exist for archiving and managing the geo-referenced data sets that are needed for Article 76 work. These tasks are common to many geographic and geoscientific endeavours, so the tools are common and well developed. Fewer proprietary tools exist for analyzing these georeferenced data sets for Article 76 purposes and for visualizing the outcome. These tasks are unique to Article 76 and hence represent a smaller market for software developers. Some coastal States have developed their own tools in house. A coastal State should seek expert advice in the selection and procurement of these tools, taking its unique circumstances into account: there is no one size fits all solution.

50 THE TEST OF APPURTENANCE The submitting State must demonstrate that a portion of its continental landmass extends beneath the sea and comprises a genuine natural prolongation The CLCS seems to prefer a geomorphological definition of the natural prolongation There may be some scope for invoking evidence to the contrary, e.g. geological or geophysical information, in defining the natural prolongation

51 SOME ILLUSTRATIONS OF THE TEST OF APPURTENANCE Islands located beyond the continental EEZs of their parent States: Ecuador and the Galapagos Islands San Felix and San Ambrosio Islands, Chile Azores and Madeira Islands, Portugal Trinidade and Martin Vaz Islands, Brazil

52 ECUADOR AND THE GALAPAGOS ISLANDS Costa Rica Colombia Ecuador

53 SAN FELIX AND SAN AMBROSIO ISLANDS, CHILE

54 AZORES AND MADEIRA ISLANDS, PORTUGAL

55 TRINIDADE AND MARTIN VAZ ISLANDS, BRAZIL

56 TRINIDADE AND MARTIN VAZ ISLANDS, BRAZIL: CLAIMED OCS LIMIT

57 NATURAL PROLONGATIONS AND THE TEST OF APPURTENANCE OFF ECUADOR Acceptable? Not acceptable?

58 THE ARCTIC OCEAN: MORPHOLOGICAL BREAKS IN NATURAL PROLONGATIONS Canada Alpha-Mendeleev Ridge Greenland Russia Lomonosov Ridge

59 THE FOOT OF THE SLOPE The point of departure for the two Formula Lines of Article 76 Defined as the point where the seabed undergoes a maximum change of gradient Not unambiguous: the FoS point may be difficult to determine with any precision, or there may be multiple contending points to choose from Important to document carefully the choice of FoS point and reasons why it was selected, in case of a query or challenge by the CLCS

60 AMBIGUITIES IN LOCATING THE FOOT OF SLOPE Two databases describing the same section of the seafloor: note multiple FOS choices in each profile, lack of agreement between profiles

61 LOCATING THE FOOT OF THE SLOPE Fitting a smooth curve to the bathymetric profile POINT OF OF MAXIMUM CHANGE OF OF BOTTOM GRADIENT

62 LOCATING THE FOOT OF THE SLOPE Fitting straight lines to the bathymetric profile POINT OF OF MAXIMUM CHANGE OF OF BOTTOM GRADIENT

63 POSSIBLE FOS PROFILES OFF ECUADOR AND THE GALAPAGOS ISLANDS

64 BATHYMETRIC PROFILE AND FOOT OF SLOPE POINTS, CARNEGIE RIDGE

65 POSSIBLE FOS PROFILES OFF ECUADOR AND THE GALAPAGOS ISLANDS

66 BATHYMETRIC PROFILE AND FOOT OF SLOPE POINT, COLON RIDGE

67 POSSIBLE FOS POINTS AND LINES

68 THE 2500 METRE ISOBATH Key to constructing one of the constraint or cutoff lines of Article 76 Requires more measurement accuracy than does the determination of FoS points May apply only partially or not at all, depending on the configuration of the seabed adjacent to the submitting coastal State

69 2500 M ISOBATH NOT APPLICABLE 2500 m isobath 2500 m M FoS 200 M limit 350 M limit

70 2500 M ISOBATH PARTIALLY APPLICABLE THE ARCTIC SITUATION 350 NM CUTOFF 2500 M ISOBATH PLUS 100 NM PARTIAL SEGMENTS OF 2500 M PLUS 100 NM 2500 M ISOBATH

71 THE GARDINER LINE The point where sediment thickness is equal to 1% of the distance back to the FoS One of the Formula Lines of Article 76 An attempt to devise a criterion that could be applied consistently to the sedimentary material lying beneath the seabed adjacent to a submitting State Based on simplifying assumptions that bear little relationship to physical reality

72 AMBIGUITIES IN LOCATING THE GARDINER LINE Upper figure: the principle. Lower figure: the practice.

73 EXAMPLE: SEABED SEDIMENT OFF ANGOLA

74 A PROFILE ACROSS THE SEDIMENT OFF ANGOLA 1% Thickness Line

75 ARTICLE 76 FORMULA LINES OFF ANGOLA

76 IN CONCLUSION If making new measurements of bathymetry and sediment thickness, choose appropriate techniques. If using available databases, review their contents carefully to verify their suitability. Choose database and GIS tools that are adequate to the tasks. Document carefully all data sets and analytical procedures. Develop and maintain an awareness of how other States are developing their own OCS limits.

77 THANK YOU!